Petroleum, also commonly referred to as oil, consists of a complex mixture of hydrocarbons of various molecular weights, plus other organic compounds. Petroleum is a naturally occurring liquid found in rock formations. It is generally accepted that oil is formed mostly from the carbon rich remains of ancient plankton after exposure to heat and pressure in the Earth's crust over hundreds of millions of years gradually transforming into oil and natural gas reservoirs. Petroleum is a vital component of the world's supply of energy as a source of providing heating and electricity. It is also used as fuel for vehicles when refined, and as a chemical feedstock in the manufacture of plastics and other commercially important organic chemicals. Worldwide consumption of oil is approximately thirty billion barrels (4.8 km3) per year, with developed nations being the largest consumers. For example, the United States consumed about 25% of the oil produced in 2007. Petroleum is found in deep underground natural rock formations and may be associated with other hydrocarbons such as natural gas.
Oil reservoirs may be located deep within the Earth's crust. As recovery technology advances, oil recovery methods are being performed in deeper locations within the Earth, most notably in offshore and deep ocean locations. For example, deep ocean drilling rigs are now drilling in water depths at or in excess of 2,000 meters. Similarly, there is much activity at land based locations.
Oil recovery may take a variety of forms and methods. For example, once a reservoir is identified, an oil well is created by drilling a long hole into the Earth. A steel pipe, known as a casing, is placed in the hole to provide structural integrity to the newly drilled well bore. Holes are then made in the base of the well to enable oil to pass into the bore, which oil is then removed by various methods. Typically, recovered oil includes various other secondary byproducts such as natural gas, inorganic compounds and water associated with it. As wells mature, various techniques are employed to extract as much oil as possible. These techniques are commonly referred to as enhanced oil recovery (EOR). One of these techniques injects treated water into a reservoir to displace the oil. This technique requires that the water is of specific quality which necessitates treatment prior to injection. Another technology being used to recover previously unrecovered oil is hydraulic fracturing. This is a technique used to create fractures in rock with a hydraulic fluid, typically water with additives, under high pressure to release trapped hydrocarbons.
Crude primary treatment techniques may comprise multistep processes. For example, a technique may include first separating byproducts from raw crude oil, followed by desalting the crude oil. The byproducts and the raw crude are separated in a device called a separator or dehydrator which removes water. There are several types of separators depending on the feed stream and the separation objectives. Crude oil, natural gas, produced water, bottom sludge which is typically sand, and other inert compounds are separated. The oil is then washed with water to remove the salts that are trapped within the crude oil, i.e., desalting. The washing removes salts and generates a wastewater stream that contains dissolved salts, suspended material, oil, benzene, ethylbenzene, toluene and xylenes (BETX), and in some cases heavy metals.
Typical crude oil separation methods generate substantial quantities of waste. Such systems can generate from as little as 5,000 barrels per day (BPD) to upwards of 300,000 BPD. Waste water is generated from the water associated with the recovered hydrocarbons as well as water used to desalt the crude oil. The characterization of the water will vary according to its content. As oil and gas production wells mature, there is an increased percentage of produced water being generated. Produced water limits the capacity of crude oil transportation by corroding conveyance systems.
Regulations related to discharge of produced water vary by the authorities involved and the location receiving the waste stream. Table 1 represents typical waste water stream and associated regulatory discharge levels.
TABLE 1PPM Range (unlessTypical Discharge limits - PPMConstituentotherwise noted)(unless otherwise noted)Free Oil & grease100-1000<15BOD100-2000150COD1000-5000 VariesTemperature25-200 deg. C.40 deg. C.Hydrogen Sulfide 0-100Ph6-8 VariesTDS50,000-300,000Varies - platform deep oceandilutionTSS100-1000150Ammonia 0-100Sulfates500-5000Heavy Metals10-200Varies by metal <1.5Silica100-2000Sodium30,000+Chloride30,000+Hardness  1000+Iron10-100Mercury1-100.01Oxygen5-10
As the amount of crude oil recovered increases and additional water based enhanced techniques are used, the amount of produced water generated also increases, thereby creating serious environmental challenges to be addressed. Issues such as the contamination of water ways such as stream, lakes, groundwater with water containing, oil, grease, hydrocarbons, metals, etc., must be prevented, some of which contaminants result in increased levels of chemical oxygen demand (COD) and biochemical oxygen demand (BOD). Moreover, typical produced water is extremely high in total dissolved solids (TDS), sometime ten times that of sea water. TDS can destroy streams, lakes and groundwater by raising salinity levels. Furthermore, EOR techniques consume large quantities of water. For example, recovery of hydrocarbons consumes substantial quantities of fresh water for production activities. As oil recovery activities from reservoirs mature and EOR activities increase, scarce water resources are taxed at an increasing rate. Hydrofracturing activities require water treated to specific criteria. Once the well is fracked, there is substantial water that is removed from the well. This is called flowback and must be treated in a similar fashion to produced water.
Various known methods of treating produced water are presently utilized. For example, produced water is sent through separate conveyance lines or combined with oil and transported to shore for treatment. Additionally, produced water is injected back into deep wells; however, this sometimes results in the water reentering the oil reserves thereby creating further problems. Moreover, produced water is treated with conventional technologies that are large, heavy and generate substantial quantities of sludge while consuming large amounts of chemicals. Often the resulting sludge is not recoverable into commercial products and must be disposed of in a land fill.
The foregoing options for treating produced water suffer from the various defects described above, e.g., expensive, complex, difficult to clean, etc. The present system and method for treating produced water provides a variety of benefits that have heretofore been lacking in known systems. For example, the present system and method recovers hydrocarbons for commercial value while treating the water for total suspended solids (TSS), oil, metals, H2S, BOD and other undesirable components. The present invention is sufficiently flexible to treat different produced water streams and can accommodate changes in those streams that may occur during operation. The present system has a small foot print and minimum weight. The present system and method generates minimum secondary waste and solids while being simple and easy to operate. The present invention requires minimum consumables and chemicals while producing treated water of a quality that allows for reuse or discharge. The present invention provides water for EOR, desalting, hydrofracturing and other production activities wherein the water is treated for the removal of contaminants such as sulfates, barium, boron, total dissolved solids, suspended solids, H2S and oxygen, and agents such as biocides are added to prevent sulfate reducing bacteria from reducing sulfates to hydrogen sulfide (H2S), for example as need in EOR use. The present invention provides secondary waste streams from EOR operations that meet or exceed discharge standards.